1. Field of the Invention
The present invention relates to a semiconductor device and a method of fabrication thereof, and more specifically to a semiconductor device and a method of fabrication thereof, in which the semiconductor device includes a capacitive structure comprising a capacitive insulating film formed with a Ta.sub.2 O.sub.5 film.
2. Description of the Prior Art
In recent ultra-large scale integrated circuit (LSI) memories many efforts are paid as an important subject to increase a capacity of a high dielectric tantalum oxide film per unit area.
Referring to FIG. 1, there is illustrated in a cross section a capacitive structure of a prior art DRAM with use of a tantalum oxide film. As illustrated in the figure, a device isolation oxide film 202 is selectively formed on the surface of a silicon substrate 201 with a gate oxide film formed thereon, and thereafter a gate electrode 203 serving also as a word line is formed together with impurity diffusion layers 205a, 205b as a source-drain region of a MOSFET formed on the surface of the silicon substrate 201. Simultaneously with the formation of the impurity diffusion layers 205a, 205b, a silicon oxide film 204 (the silicon oxide film includes a gate oxide film, a spacer oxide film, and a silicon oxide film for an interlayer insulating film) is formed.
A node contact is opened through the silicon oxide film 204 on the surface of the impurity diffusion layer 205a and a storage node electrode 206 comprising polycrystalline silicon is connected to the impurity diffusion layer 205a. Further, a capacitive insulating film 207 comprising a tantalum oxide film covering the surfaces of the storage node electrode 206 and the silicon oxide film is formed together with the formation of a plate electrode 208. The capacitive insulating film 207 excepting a portion of the same just under the plate electrode 208 is removed by etching. For the plate electrode 208, there are employed an aluminum (Al) metal film or a tungsten (W) film as a high melting point metal film, and a molybdenum (Mo) film. For the formation of the aluminum metal film there is employed vacuum deposition or sputtering, and for the formation of the tungsten or molybdenum film being the high melting point metal film there is employed sputtering.
The prior art capacitive structure described above however has the following difficulties: First, provided the plate electrode comprises the aluminum metal film, there occurs a reaction between aluminum and tantalum oxide upon film formation to cause a leakage current in the tantalum oxide film to be increased.
Contrarily, provided the plate electrode comprises the high melting point metal film such as a tungsten film, the leakage current through the tantalum oxide film is reduced compared with the case where the aluminum metal film is employed This originates from a fact that use of the high melting point metal film prevents the reaction with tantalum oxide from being produced. However, even the use of such a high melting point metal film as the plate electrode fails to ensure a highly reliable tantalum oxide film because there is left a problem in view of a practical current level.